A power supply unit generates a desired potential of output voltage by boosting or dropping the power supply voltage. The power supply unit is therefore a voltage converter which converts the power supply voltage supplied from the outside into desired voltage.
In the case of a step-down type voltage converter, a switching element coupled to the power supply voltage performs the ON/OFF switching operation according to the potential of the output voltage, and generates output voltage having a desired potential for an output terminal by intermittently outputting current to the output terminal. An inductor is disposed between the switching element and the output terminal, and current smoothed by the inductor is output to the output terminal. A load circuit is coupled to the output terminal, and voltage having a desired potential is output to the load circuit. The potential of the output voltage fluctuates according to the power consumption of the load circuit, and the voltage converter performs the switching operation so as to minimize the fluctuation (ripple).
In the case of a step-up type voltage converter as well, a switching element is disposed between inductor coupled to the power supply voltage and a reference potential, such as a ground, and current is intermittently supplied to the inductor by the ON/OFF switching operation of the switching element, the current is output to the output terminal by electromagnetic energy stored in the inductor, and output voltage having a desired potential, which has been boosted to be higher than the power supply voltage, is generated for the output terminal. In this case as well, a load circuit is coupled to the output terminal, and voltage having a desired potential is supplied to the load circuit. The potential of the output voltage fluctuates according to the power consumption of the load circuit, and the voltage converter performs the switching operation so as to minimize the fluctuation.
The voltage converter has an inductor for smoothing the output current, as mentioned above, and a capacitor for smoothing the output voltage is coupled to the output terminal. A standard voltage converter is a DC-DC converter which converts a DC voltage into another DC voltage.
In such a voltage converter, the inductor and the capacitor are large and expensive, and are normally externally coupled to a power supply chip integrating a switching element and a control circuit for controlling the switching element. In order to provide a sufficient smoothing function, the inductance of the inductor and the capacitance of the capacitor must be large, and for this reason the external dimensions of the conductor and the capacitor are large.
The inductance of the inductor and the capacitance of the capacitor may be decreased by increasing the switching frequency. In order to increase the switching frequency however, a power MOSFET, which operates at high-speed, is necessary for the switching element, and the chip size of such a power MOSFET is large. Furthermore, it is difficult to implement an inductor and a capacitor of which loss is small even when operating at high frequency, and cost thereof is high.
To solve this problem, a multiphase type voltage converter was proposed, where a voltage converter is constituted by a plurality (N) of sub-voltage converters, and these sub-voltage converters perform the switching operation in N phase. If the multiphase type is used, the frequency of each sub-voltage converter may be decreased, and the requirements for the power MOSFET, the inductor and the capacitor may be relaxed. In other words, the size of the power MOSFET may be decreased by decreasing the frequency. Since the voltage converter has a plurality (N) of sub-voltage converters, the overall inductance is set to a demanded value even if the inductance of the inductor of each sub-voltage converter is decreased to 1/N. The following non-patent documents all disclose a multiphase type voltage converter.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-260992
[Non-Patent Document 1] “A DLL Based Multiphase Hysteretic DC-DC Converter”, P. Li, ISQED, 2007, pp. 98
[Non-Patent Document 2] “A Hysteretic Control Method for Multiphase Voltage Regulator”, K. Lee, IEEE Power Electronics, vol. 24, No. 12, (2009), pp. 2726
[Non-Patent Document 3] “A Multiphase DC/DC Converter with Hysteretic Voltage Control and Current Sharing”, W. Gu, APEC, 2002, pp. 670
[Non-Patent Document 4] “A 480-MHz Multi-Phase Interleaved Buck DC-DC Converter with Hysteretic Control”, G. Schrom, IEEE 35th Power Electronics Specialist Conf., (2004), pp. 4702
[Non-Patent Document 5] “Multiphase Voltage-Mode Hysteretic Controlled VRM with DSP Control and Novel Current Sharing”, J. A. Abu-Qahouq, APEC, 2002, pp. 663
[Non-Patent Document 6] “New Digital Control Architecture Eliminating the Need for High Resolution DPWM”, J. Li, PESC, 2007, pp. 814
The multiphase type voltage converter however, must control the switching of the plurality of sub-voltage converters in N phases, and perform complicated pulse width modulation (PWM) and pulse frequency modulation (PFM), hence the control circuit for controlling the switching becomes complicated and large in terms of circuit scale. Particularly in the case of decreasing the sizes of the inductor and the power MOSFET of an individual sub-voltage converter and making the output capacitor small by increasing the number of sub-voltage converters, a large scale control circuit of each sub-voltage converter causes the overall circuit scale to increase.